Papillomavirus infections occur in a variety of animals, including humans, sheep, dogs, cats, rabbits, snakes, monkeys and cows. Papillomaviruses infect epithelial cells, generally inducing benign epithelial or fibroepithelial tumors at the site of infection. Papillomaviruses are species specific infective agents; a human papillomavirus cannot infect a non-human.
Papillomaviruses are small (50–60 nm), nonenveloped, icosahedral DNA viruses what encode up to eight early and two late genes. The open reading frames (ORFs) of the virus are designated E1 to E7 and L1 and L2, where “E” denotes early and “L” denotes late. L1 and L2 code for virus capsid proteins. The early genes are associated with functions such as viral replication and cellular transformation.
In humans, different HPV types cause distinct diseases, ranging from benign warts (for examples HPV types 1, 2, 3) to highly invasive genital and anal carcinomas (HPV types 16 and 18). At present there is not a satisfactory therapeutic regimen for these diseases.
Immunological studies in animals have shown that the production of neutralizing antibodies to papillomavirus antigens prevents infection with the homologous virus. However, development of a vaccine has been hindered by the difficulties associated with culture of the papillomavirus in vitro.
Vaccination is an effective form of disease prevention and has proven successful against several types of viral infection. However, to date, attempts to generate an effective HPV vaccine have not been entirely successful.